When producing energy, water flowing from a higher level to a lower level drives a hydraulic turbine is coupled to an alternator that delivers energy to an electricity grid via a very high tension/medium tension transformer (abbreviated below as a VHT/MT transformer), serving to raise voltage. When consuming energy, the grid delivers electrical energy to a motor through the same voltage transformer, and the motor drives a pump that raises the water to the higher level. The machines are excited by means of static thyristor generators fed via medium tension/low tension transformers (abbreviated to MT/LT) which are connected to the VHT/MT transformer and which are therefore called take-off transformers. The pumping station is connected to the grid by means of a circuit breaker.
Before starting any machine, motor or alternator depending on the selected mode of operation, it is necessary to put the VHT/MT transformer under tension so as to enable it to feed the take-off transformer corresponding to the machine that is to be used.
Because of the low power of the take-off transformer, this takes place as though the VHT/MT transformer were put under tension while unloaded.
Putting an unloaded transformer under tension gives rise to a surge current whose peak value depends on the voltage value at the instant of connection. If connection takes place while the alternating voltage is passing through an extreme value, then the surge current is low, whereas if connection takes place when the grid voltage is in the vicinity of zero, then the surge current may reach high values, of the order of five times to ten times the nominal current of the transformer. Such an overload has the effect of fatiguing the transformer because of the electromagnetic forces generated and also accelerates the aging of the insulation. Such insulation aging can be further accelerated if parallel ferro-resonant phenomena occur.
Unfortunately, the transformer of a pumping station may be switched several times a day, such that unless it is very considerably overdimensioned, it is subjected to rapid degradation.
An object of the invention is to provide a pumping station with means for limiting the surge current in its transformer at the moment it is engaged.
Proposals have been made, in particular in the journal Electra, No. 94, May 1984, in an article entitled "Problems relating to the magnetic circuits of transformers and of reaction coils" by H. Kan, to provide the circuit breaker with means for inserting a resistance of 4,300 ohms in parallel with its contacts for a period of 25 milliseconds on closing, or a resistance lying in the range 3,000 ohms to 7,000 ohms for a period of not less than 13 milliseconds.
The Applicant has observed that the insertion time is an essential parameter in implementing this technique. If the insertion time is too short, then the current is insufficiently clipped; if the duration is too long, then the energy absorbed by the resistance is too great and may lead to rapid destruction thereof.
According to a characteristic of the invention, the resistance of the insertion resistor lies in the range 500 ohms to 100,000 ohms, and the insertion duration lies in the range 15 milliseconds to 19 milliseconds.
Another way of limiting the surge current in a pumping station is based on the observation that the surge current is smaller the nearer the grid voltage is to an extreme value at the instant when the transformer is engaged.
According to the invention, the pumping station includes means acting on the control of the circuit breaker to allow it to close on the unloaded transformer only at an instant that is not more than 1.2 milliseconds on either side of the instant at which the grid voltage passes through an extreme value. This technique can be used on its own or in combination with the preceding technique.
By using this technique, the current through the transformer need not exceed twice the nominal current of the transformer, as has been shown by calculations performed by the Applicant.
When applying this solution, the voltage wave applied to the transformer is at a maximum. Large amplitude oscillations at a frequency of several MHz and having steep wave fronts appear in the metal-clad station and this is harmful to the transformer. According to a characteristic of the invention, to avoid this drawback, each of the phase conductors connecting the transformer to the circuit breaker is grounded through a circuit including a capacitor and a resistor in series. In a metal-clad configuration where the capacitor and the resistor are placed in a metal tube, the resistance of the resistor should be equal to the characteristic impedance of said tube. The capacitance of the capacitor should be not less than 2 nanoFarads.